I personally use BH pioneer 505, i dont know anything about the 503, but from what i have read, the main difference between the 503 and 505 is that the 505 has the "no motion-all metal" mode (AKA Pin point). this is a feature that i dont think i could live without. if you are seriously concidering a BH pioneer series, i suggest the 505. now as for beach hunting, i have not had much luck hunting beaches with my 505, i dont think it liked the mineralization in the sand, but, to be honest, i probably didnt adjust the discrimination properly. i usually stick with parks and schools and my 505 works fantastic for that.